"Quantum mechanics is certainly imposing. But an inner voice tells me that this is not yet the real thing. The theory says a lot, but does not bring us any closer to the secrets of the "Old One." I, at any rate, am convinced that He is not playing at dice." Albert Einstein

Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of the dual particle-like and wave-like behaviour and interaction of matter and energy.

It departs from classical mechanics primarily at the atomic and sub-atomic scales, the so-called quantumrealm. In advanced studies of quantum mechanics some of the behaviours are macroscopic, but these emerge only at extremely low or extremely high energies or temperatures.

The term was coined by Max Planck, and derives from the observation that some physical quantities can be changed only by discrete amounts, or quanta, as multiples of the Planck constant, rather than being capable of varying continuously or by any arbitrary amount. For example, the angular momentum, or more generally the action, of an electron bound into an atom or molecule is quantized. Although an unbound electron does not exhibit quantized energy levels, one which is bound in an atomic orbital has quantized values of angular momentum. In the context of quantum mechanics, the waveâ€“particle duality of energy and matter and the uncertainty principle provide a unified view of the behavior of photons, electrons and other atomic-scale objects.

The mathematical formulations of quantum mechanics are abstract. Similarly, the implications are often counter-intuitive in terms of classical physics. The centerpiece of the mathematics of quantum mechanics is the wavefunction, which describes the probability amplitude of the position and momentum of a particle. Mathematical manipulations of the wavefunction usually involve the bra-ket notation, which requires an understanding of complex numbers and linear functionals. The wavefunction treats the object as a quantum harmonic oscillator and the mathematics is akin to that of acoustic resonance.

Many of the results of quantum mechanics do not have models that are easily visualized in terms of classical mechanics; for instance, the ground state in the quantum mechanical model is a non-zero energy state that is the lowest permitted energy state of a system, rather than a traditional classical system that is thought of as simply being at rest with zero kinetic energy.

Fundamentally, quantum mechanics is an attempt to explain the peculiar behaviour of matter and energy on the subatomic level - an attempt that has produced more accurate results than classical physics, in terms of predicting how individual particles behave. But many unexplained anomolies nevertheless remain. (underline added)

Historically, the earliest versions of quantum mechanics were formulated in the first decade of the 20th century, around the time that atomic theory and the corpuscular theory of light as interpreted by Einstein first came to be widely accepted as scientific fact; these latter theories can be viewed as quantum theories of matter and electromagnetic radiation. Quantum theory was significantly reformulated in the mid-1920s, away from the old quantum theory towards the quantum mechanics formulated by Werner Heisenberg, Max Born, Wolfgang Pauli and their associates, based upon acceptance of the Copenhagen interpretation of Niels Bohr, as a science of probabilities. By 1930 quantum mechanics had been further unified and formalized by the work of Paul Dirac and John von Neumann, with a greater emphasis placed on measurement, the statistical nature of our knowledge of reality, and philosophical speculations about the role of the observer. (wikipedia)